Superconducting lanthanum nickel oxides with bilayered and trilayered crystal structures

La₃Ni₂O₇exhibits superconductivity belowTc≃80 K under high pressures exceeding approximately 14 GPa. The discovery of this superconductivity immediately attracted considerable attention because of its high transition temperature and its structural resemblance to high-Tccuprates, which subsequently led to the discovery of superconductivity in La₄Ni₃O₁₀. These compounds belong to the Ruddlesden-Popper phases, consisting of double- and triple-layered NiO₂square lattices separated by LaO rock-salt slabs. Research on these systems has rapidly advanced along three major directions, as observed in other prominent families of superconductors such as the cuprates and iron arsenides: expansion of the chemical variety of compounds, enhancement ofTcthrough elemental substitution, and elucidation of the superconducting mechanism. These closely interconnected challenges continue to drive progress in the field. Clarifying the pairing mechanism poses a particular difficulty, as key experiments must be conducted under high pressures. This situation underscores the importance of developing nickel oxides that exhibit superconductivity at much lower pressures, ideally at ambient pressure, which would substantially broaden the scope for chemical tuning and detailed physical characterization. In this context, it is timely and meaningful to summarize the current state of knowledge. Here, we focus on sample synthesis and characterization, which are already well established and often decisive for progress in unconventional superconductors, while also providing a brief overview of the electronic properties reported to date.